Skip composer PHP requirement

I’ve found the option:

composer install --ignore-platform-reqs

Ignore platform requirements (php & ext- packages).

Edit: You can skip the platform checks with this, but Composer will fetch packages based on given PHP version then. So when you need composer to also emulate a PHP version during depedency resolving, you can (and should!) use this in your composer.json:

{
    "config": {
       "platform": {
           "php": "5.6.6"
       }
    }
}

https://getcomposer.org/doc/06-config.md#platform